Finally, I think it's better to improve filters (and not do that in rmodifiers), since filter already has needed info to select lines: list of buffers, tags, regex.

The default action of a filter would be "hide line", and another action would be "change line", using another regex.

And last thing, format of filters in config file could be improved to have real options for filters. So that a filter option could be modifier with /set (no need to remove/add filter to change something).

I've experimented a bit with /rmodifier, it almost does what I want but not quite. It appears to allow matched expressions to be nullified, censored or left as is; it does not allow the the format or content to be altered.

I've experimented a bit with /rmodifier, it almost does what I want but not quite. It appears to allow matched expressions to be nullified, censored or left as is; it does not allow the the format or content to be altered.

The situation is I get sent lengthy messages from the server, for example:

For me, such task should not be done by /filter, which is just a filter to hide lines.

To change lines with regex, look at rmodifier plugin, it's purpose of this plugin: use regex to change content of modifier messages.
Try /help rmodifier
It was primarly designed to hide password displayed in input, but I think it can work to modify any message displayed.

It would be very useful if instead of just dropping matched items, /filter could perform a reg-ex substitution on them instead. I'd like to use this to filter lengthy server messages into something more space efficient, but I'd imagine this would have plenty of other uses too.